Reduce number of edge rings
In the upcoming 2.71 release there is a better way to achieve this.


Select edge ring (Ctrl+Alt+RMB)
Checker De-deselect, See note below!
Select -> Edge Loops
Mesh -> Delete -> Edge Loops


Note:

In 2.71 support has been added to deselect alternate edge-ring, See: https://developer.blender.org/T40324
This is detected automatically.



Previous answer, obsoleted by the method above

This is possible but a little cumbersome


In vertex mode select a loop running along the length
Select -> Checker De-seelct
Hide
Set edge mode
Unhide
Deselect the vertical loop
Select -> Edge Loop
Mesh -> Delete -> Edge Loop




Footnote, it may be worth adding a skip option to edge-ring select, which would make this only 2 steps.